2.1 Skeletal system and bone structure
5 min read•august 14, 2024
The skeletal system is the body's framework, providing and . It consists of 206 bones, each with specific functions. From the skull guarding the brain to the vertebrae protecting the spinal cord, bones are crucial for our structure and movement.
Bone tissue is a dynamic structure, constantly remodeling to adapt to stress and repair damage. This process involves specialized cells like osteoblasts and osteoclasts, working together to maintain bone strength. Understanding bone composition and remodeling is key to optimizing skeletal health.
Human Skeletal System
Bones and Their Functions
Top images from around the web for Bones and Their Functions
The Skull · Anatomy and Physiology View original
Is this image relevant?
Divisions of the Skeletal System · Anatomy and Physiology View original
Is this image relevant?
The Skeletal System | Biology for Majors II View original
Is this image relevant?
The Skull · Anatomy and Physiology View original
Is this image relevant?
Divisions of the Skeletal System · Anatomy and Physiology View original
Is this image relevant?
1 of 3
Top images from around the web for Bones and Their Functions
The Skull · Anatomy and Physiology View original
Is this image relevant?
Divisions of the Skeletal System · Anatomy and Physiology View original
Is this image relevant?
The Skeletal System | Biology for Majors II View original
Is this image relevant?
The Skull · Anatomy and Physiology View original
Is this image relevant?
Divisions of the Skeletal System · Anatomy and Physiology View original
Is this image relevant?
1 of 3
- The skeletal system consists of 206 bones that provide structural support, protection of vital organs, attachment points for muscles to enable movement, storage of minerals and fats, and production of blood cells
- The skull is comprised of 22 bones that protect the brain and support the structures of the face
- The cranium includes the frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones
- The face includes the mandible, maxilla, zygomatic, nasal, lacrimal, and palatine bones
- The vertebral column consists of 33 vertebrae separated into cervical, thoracic, lumbar, sacral, and coccygeal regions
- It protects the spinal cord, supports the head, and serves as an attachment point for ribs, muscles, and ligaments
- The thoracic cage is made up of 12 pairs of ribs, the sternum, and 12 thoracic vertebrae
- It protects vital organs in the chest cavity, including the heart and lungs
Limb and Girdle Bones
- The pectoral girdle consists of the clavicle and scapula, which provide attachment points for muscles that move the upper limbs
- The upper limbs contain the humerus, radius, ulna, carpals, metacarpals, and phalanges
- The pelvic girdle is formed by the hip bones (ilium, ischium, pubis) and sacrum
- It supports the weight of the upper body, protects reproductive and digestive organs, and provides attachment points for lower limb muscles
- The lower limbs contain the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges
Bone Tissue Composition and Structure
Bone Composition
- Bone is a connective tissue composed of a mineralized extracellular matrix and specialized cells: osteoblasts, osteocytes, and osteoclasts
- The extracellular matrix consists of organic and inorganic components
- The organic component is primarily type I collagen fibers, which provide tensile strength and flexibility
- The inorganic component is mainly hydroxyapatite crystals, which provide compressive strength and rigidity
Bone Structure
- Bone tissue is organized into two types of bone: cortical (compact) and trabecular (spongy)
- is dense and forms the outer layer of bones
- is porous and found in the interior of bones, particularly at the ends of (femur, humerus)
- Cortical bone is composed of osteons (Haversian systems), which are cylindrical structures with a central Haversian canal surrounded by concentric lamellae
- Osteocytes are located in lacunae between lamellae and communicate through canaliculi
- Trabecular bone consists of a network of thin, interconnected trabeculae, which provide structural support and flexibility
- The spaces between trabeculae are filled with , which produces blood cells (red blood cells, white blood cells, platelets)
- The outer surface of bones is covered by a fibrous connective tissue called the periosteum, which contains blood vessels, nerves, and osteoblasts
- The inner surface of bones is lined by the endosteum, a thin layer of cells that includes osteoblasts and osteoclasts
Bone Remodeling and Stress
Bone Remodeling Process
- is a continuous process of bone resorption and formation that helps maintain bone strength, repair microdamage, and adapt to mechanical stresses throughout life
- The bone remodeling process involves the coordinated actions of osteoclasts (bone-resorbing cells) and osteoblasts (bone-forming cells)
- Osteocytes, the most abundant cells in bone, act as mechanosensors and orchestrate the remodeling process
- The remodeling cycle consists of five stages: activation, resorption, reversal, formation, and quiescence
- During activation, osteocytes detect mechanical or biochemical signals and recruit osteoclasts to the remodeling site
- In the resorption phase, osteoclasts break down and remove old or damaged bone
- The reversal phase marks the transition from resorption to formation, with osteoblasts being recruited to the site
- During formation, osteoblasts synthesize new bone matrix (osteoid) and regulate its mineralization
- Finally, in the quiescence stage, the newly formed bone surface remains inactive until the next remodeling cycle
Mechanical Stress and Hormonal Regulation
- Bone remodeling is influenced by mechanical stress, with bone formation being stimulated by increased loading and bone resorption being promoted by decreased loading or disuse (Wolff's law)
- This adaptive response allows bones to optimize their structure and strength in response to the mechanical demands placed upon them (weight-bearing exercise, resistance training)
- Hormones, such as parathyroid hormone, calcitonin, and estrogen, as well as local factors like cytokines and growth factors, also regulate the bone remodeling process
- Imbalances in bone remodeling can lead to disorders such as (excessive bone resorption) or osteopetrosis (insufficient bone resorption)
- Maintaining a balance between resorption and formation is crucial for overall bone health and preventing fractures
Axial vs Appendicular Skeleton
Axial Skeleton
- The axial skeleton forms the central axis of the body and consists of 80 bones
- It includes the skull, vertebral column, ribs, and sternum
- The primary functions of the axial skeleton are to protect the brain and spinal cord, provide support for the head and trunk, and serve as attachment points for muscles
- Bones of the axial skeleton are generally more rigid and less mobile compared to the bones of the appendicular skeleton
Appendicular Skeleton
- The appendicular skeleton is composed of 126 bones and includes the bones of the upper and lower limbs, as well as the pectoral and pelvic girdles
- The main functions of the appendicular skeleton are to enable movement, provide attachment points for muscles, and support the weight of the body
- The pectoral girdle, consisting of the clavicles and scapulae, connects the upper limbs to the axial skeleton
- The pelvic girdle, formed by the hip bones and sacrum, connects the lower limbs to the axial skeleton
- Bones of the appendicular skeleton are adapted for greater range of motion and mechanical loading (lifting objects, running)
Skeletal System Integration
- While the axial skeleton provides stability and protection for vital organs, the appendicular skeleton allows for a wide range of movements and interactions with the environment
- The axial and appendicular skeletons work together to provide structural support, protection, and movement for the body
- They also play important roles in mineral homeostasis (calcium and phosphorus storage) and hematopoiesis (blood cell production in bone marrow)
Key Terms to Review (21)
Articular Cartilage: Articular cartilage is a smooth, white tissue that covers the ends of bones where they come together to form joints. It plays a crucial role in reducing friction and absorbing shock during joint movement, ensuring smooth and pain-free mobility. This specialized type of cartilage provides a low-friction surface for joint articulation and helps distribute loads across the joint surfaces, contributing to overall joint health and function.
Bone marrow: Bone marrow is a spongy tissue located within the cavities of bones, primarily responsible for the production of blood cells. It plays a critical role in the skeletal system by providing a site for hematopoiesis, the process through which red blood cells, white blood cells, and platelets are produced. In addition to its blood-forming function, bone marrow also contains adipose tissue and stromal cells that support hematopoiesis and contribute to the overall health of the skeletal system.
Bone Mineral Density: Bone mineral density (BMD) refers to the amount of mineral matter per square centimeter of bone, primarily calcium and phosphorus, which are essential for bone strength and integrity. BMD is a crucial indicator of bone health, as it reflects the balance between bone resorption and formation. Higher BMD values generally indicate stronger bones, while lower values can signal an increased risk for fractures and osteoporosis.
Bone remodeling: Bone remodeling is a continuous process where old bone tissue is replaced by new bone tissue, maintaining bone strength and integrity. This dynamic process involves the coordinated action of cells called osteoclasts, which break down bone, and osteoblasts, which build new bone. The balance between these activities is crucial for the maintenance of skeletal health, adaptation to mechanical loads, and response to hormonal changes.
Cortical bone: Cortical bone, also known as compact bone, is the dense and hard outer layer of bone that provides strength and support. It forms the outer shell of all bones and is critical for protecting inner structures, providing mechanical support, and facilitating movement through its interaction with muscles.
Flat Bones: Flat bones are a category of bones characterized by their thin, flattened shape, which provides protection to vital organs and serves as a site for muscle attachment. These bones are primarily composed of two layers of compact bone surrounding a layer of spongy bone, creating a lightweight yet strong structure. They are essential components of the skeletal system, contributing to overall stability and function.
Loading Response: The loading response refers to the initial phase of weight acceptance during the gait cycle, where the body's weight is transferred onto the leading leg as it makes contact with the ground. This phase is crucial for shock absorption and stability, involving complex interactions between muscles, joints, and the skeletal system to support movement and maintain balance.
Long Bones: Long bones are a category of bones characterized by their elongated shape, typically being longer than they are wide. They play a crucial role in movement and support, serving as levers that facilitate actions like walking, running, and lifting. Long bones also house bone marrow, which is essential for producing blood cells, and they contribute to the overall structure and strength of the skeletal system.
Mechanotransduction: Mechanotransduction is the process through which cells convert mechanical stimuli into biochemical signals, impacting cellular behavior and function. This fundamental mechanism plays a crucial role in the skeletal system, as bones and cartilage respond to mechanical forces through remodeling, growth, and repair. Understanding how mechanotransduction works is essential for grasping how physical activity influences bone health and overall skeletal integrity.
Mineral Storage: Mineral storage refers to the ability of bones to act as reservoirs for essential minerals, primarily calcium and phosphorus, which are crucial for various bodily functions. This process ensures that these minerals are available for metabolic activities, bone formation, and maintaining overall mineral balance in the body. When the body needs these minerals for functions like muscle contraction or nerve signaling, bones can release them into the bloodstream.
Osteogenesis: Osteogenesis is the biological process of bone formation, which involves the development and mineralization of bone tissue. This process is crucial for the growth, repair, and maintenance of bones throughout a person’s life. It plays a significant role in the skeletal system, influencing how bones grow in size and strength, and ensuring that the skeletal structure remains resilient against physical stress and injury.
Osteoporosis: Osteoporosis is a medical condition characterized by decreased bone density and quality, leading to an increased risk of fractures. This condition often results from an imbalance between bone resorption and formation, where the body loses too much bone or doesn't make enough new bone. Understanding osteoporosis is crucial as it directly relates to the skeletal system, highlighting how bone structure can weaken and how lifestyle factors such as diet and exercise can influence bone health.
Peak Bone Mass: Peak bone mass refers to the maximum amount of bone tissue an individual can attain at the end of skeletal maturation, typically occurring in early adulthood. This critical stage plays a significant role in determining overall bone health and risk for osteoporosis later in life, as higher peak bone mass is associated with a lower risk of fractures. Factors influencing peak bone mass include genetics, nutrition, physical activity, and hormonal levels during growth periods.
Protection: Protection refers to the role of the skeletal system in safeguarding vital organs and structures within the body. This includes providing a barrier against mechanical injury and damage from external forces, ensuring the integrity of essential systems such as the nervous and circulatory systems. The bones act as a shield, supporting overall bodily function and allowing for movement while maintaining structural stability.
Short Bones: Short bones are a category of bones that are roughly equal in length, width, and thickness, giving them a cube-like appearance. These bones provide stability and support while allowing for some motion, making them crucial in areas of the body where precise movement is essential. Examples include the bones of the wrist and ankle, which help in complex movements and weight-bearing activities.
Sir Charles Bell: Sir Charles Bell was a prominent 19th-century Scottish surgeon and anatomist known for his significant contributions to the understanding of the nervous system, particularly the structure and function of spinal nerves. His work laid the groundwork for further discoveries in neurology and helped establish important concepts in anatomy, especially related to sensory and motor pathways.
Stress Fracture: A stress fracture is a small crack in a bone that develops due to repetitive force or overuse, often resulting from high-impact activities. This condition is commonly seen in athletes and active individuals, as the bones experience more strain than they can tolerate. Factors such as bone density, training intensity, and technique can significantly influence the risk of developing stress fractures.
Support: Support refers to the structural and functional role that the skeletal system plays in maintaining the body's shape and stability. It provides a framework that enables the body to stand upright, facilitates movement through muscle attachment, and protects vital organs. The strength and integrity of bones are crucial for this support, influencing overall health and physical performance.
Synovial Joint: A synovial joint is a type of joint that allows for a wide range of motion, characterized by the presence of a synovial cavity filled with fluid, articular cartilage covering the bone surfaces, and a joint capsule that surrounds the joint. These joints facilitate movement between bones while providing lubrication, cushioning, and shock absorption, which are essential for daily activities and athletic performance.
Trabecular bone: Trabecular bone, also known as cancellous or spongy bone, is a type of bone tissue characterized by a porous structure that consists of a network of trabeculae or small, rod-like elements. This spongy appearance is crucial for reducing the weight of the skeleton while still providing support and strength, especially at the ends of long bones and in the interior of other bones. Trabecular bone plays an essential role in mineral storage and is highly vascularized, making it important for metabolic processes within the skeletal system.
Wolf's Law: Wolf's Law states that bone in a healthy person or animal will adapt to the loads under which it is placed. This means that bones grow and remodel in response to the forces exerted on them, becoming denser and stronger when subjected to increased stress. This adaptive response is crucial for understanding how bones develop, heal, and respond to physical activity.